Discussion about Parameters of Active RFID Antenna Design

locpeeverElectronics - Devices

Nov 27, 2013 (3 years and 6 months ago)

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Discussion
about

Parameters
of

Active RFID Antenna Design


Che
-
Wei Su


Department of Computer and Communication Engineering, Chienkuo Technology University

Changhua,
Taiwa
n
.

, R.O.C.

(jwsue@cc.ctu.edu.t
w



Abstract


Discussions

of a
dipole

antenna design
for
achieving
application of radio
frequency

identification
(RFID)
is presented.
Parameters of the RFID
antenna
design include
a thick
ness of the
substrate
, the size of
antennas,
and adjustment

mechanism

of antenna
impedance match
.
A

prototype constructed
for
RFID

operat
es

in the

9
2
2

9
28

M
Hz

band. Details of the
antenna design are presented
.

Keyword:

antenna,
RFID
,
dipole
antennas
.


I.

I
NTRODUCTION

There has been an enormous upsurge in the popularity of
RFID system in recent year. Develop of RFID systems have
taken this
development

into
account
, with the result that
contactless systems are
available

on the market. For
example, various fields of application include ticketing,
animal

identification, industrial automation or access control
[1]
. One of the most imp
ortant characteristics of RFID
system is the operating frequency and the resulting range of
the system. Active RFID
incorporates

a battery, which
supplies all or part of the power for the operation of a
microchip. This RFID system is operated for long rang
e and
the distance of data
transmission

is larger than 1 m. For the
long
-
range system, data are transmitted by electromagnetic
wave rather
than

inductive couple. Therefore, features of
antennas will determine
the

operating efficiency of RFID
system [2].


Fig.
1 T
he
dipole

antenna with

2wavelength


In contrast, a 900 MHz RFID antenna is about one half
wavelength long, or about 14 cm

as shown in Fig. 1
. A half
wavelength antenna is known as a dipole

[3]
. A dipole is a
very good radiator. RFID tags using
dipoles at 900 MHz
have a range of up to 10 meters. This works well for large
pallets of product
.
In such a RFID antenna design,
Due to
the restricted dimension for
RFID
applications, the straight
arms of a dipole antenna must be bended to form a
meander
-
l
ine
dipole
antenna
.
Antenna gains
, field

patterns and
impedance matching are changed from the conventional
dipole antenna to meander
-
line dipole antennas. Therefore,
p
arameters of antenna design must be carefully
analyzed

and
procedure of antenna design de
scribed in
detail

for active
RFID

antennas
.

To demonstrate the proposed
dipole

antenna, a prototype suitable for
RFID

operation in the
900

GHz band (
92
2

928

GHz) is constructed and studied

and
d
etails of the design considerations for the proposed antenna
a
re
also
presented
.


Fig.
2

(a) Geometry of the
LP dipole

antenna
,
(
b
) Geometry of the
proposed
LP

antenna
, and (c) Dimensions of the substrate.



II.

B
ASIC
G
UIDELINES

Figure
2
(
b
)
and
2
(c)
show
s

the geometry of the proposed
dipole

antenna with a short
ed

elemen
t for impedance match
and dielectric of substrate is

r

for
supporting

the proposed
dipole

antenna
, respectively
.
Also, t
he
meander
ed

dipole
antenna is
shown in Figure
2
(
a
)
.

The
meander
-
line

dipole antenna has

capacitive

impedance, its impedance decreases
with
in

high frequency

range

[3]
. On the source port, impedance match can be
obtained with series lumped element. In this case, the
additional component will
increases in

cost
s

of
RFID
antenna

designs
. Therefore, integration of antennas with
matched mechani
cs is the best solution
for

this

problem. By
circuit theory, the equivalent impedance
is
increase
d

in the
capacitive component shunted with an inductive element. In
figure
2
(b), the shorted metal strip mounted on the source
port
formed

a current
loop;

this

loop is equivalent
as
an

inductor. By fine adjustment of the size of a current
-
carrying

loop, the good impedance matching can easily be obtained

at
the source port
.


R
0
C
0
L
0
/2
L
0
/2
R
0
C
0
L
0
/2
L
0
/2
L
m
(a)
(b)

Fig.
3

(a)
Equivalent circuit

of the
LP dipole

antenna

and
(
b
)
Equivalent circuit

of the proposed
antenna
.


Fig
.

3
(a) show
s

the

e
quivalent circuit

of the
meandered

dipole

antenna
and this impedance is expressed as the
following equation:












0
0
0
0
1
C
L
j
R
Z





(1)

where Z
0

= R
0

at resonant o
peration and the
R
0

is a low
value

which

is

less than 50

. In Fig. 2(b), the
impedance

of
t
h
is

circuit

shunted with an
inductance

component
L
m

will
increase in magnitude of equivalent resistor,

the

impedance
is expressed as the following equation:




0
0
0
0
2
0
0
0
1
1
L
j
C
R
j
C
L
C
R
j
C
L
Z
m
m









.


(2)


Fig
.

4

show
s

the

e
quivalent circuit

of the
meandered

dipole

antenna
has
low

capacitive
impedance

and that of
a

dipole
antenna with a shorted loop

has high inductive.
With fine
adjustment of
L
m
,
the

resonate

resistor approached t
o
50

.

In Fig.
2
(b), the shorted loop metal is equivalent
with
inductor
L
m
.
Therefore, the size
adjustment
of this metal
loop is used to
achieve

the impedance match of the proposed
antenna

and
w

is

a
parameter

of
the

impedance

matching
.


In mean time, the indu
ctor
L
m

will reduce the effect length
of
a
dipole antenna. Therefore,
impedance
match will
achieve

with the
resonant

frequency shifting to high value
.

For reducing the
resonate

frequency of the
impedance
-
matched, the length of a dipole antenna must be incr
eased.
There are

two
parameters

is used to

lengthen

a dipole
antenna

by
lengthen
ing
A
rm
a

and
A
rm
b
. Arm
a

has more a
lengthening
sensitivity

than
Arm

b
. With increasing in the
same frequency range,
parameter

b

is
lengthier

than
parameter
a
. When the shift
ing frequency
has

the larger
scale
, it is
priority

to increase in

Arm a
. For
the center
frequency

with

925 MHz,
b

is used as a parameter of the fine
adjustments.



Fig.
4

Smith chart

of
a

dipole antenna with a shorted loop

and a
meandered

dipole antenna
.

III.

S
IMULATED
R
ESULTS


A prototype of the proposed antenna for
RFID

operation
was constructed and studied. The design dimensions are
given in Figure 1, which were selected by following the
design considerations described in
the above s
ection.
A

thick
ness
h

of
the
substrate
is
1.6

mm

and dielectric constant

r

is equal to 4.4
. S
ize
s of

g
round plane
s

of meander
-
line
dipole

antenna and
the proposed
prototype
antenna were
also chosen
1
44

×
34

mm
2

and

1
44

×
37

mm
2
,

respectively
.

T
he probe
-
pin length of the probe feed

was
0.
1 mm only

for
the layout of RFID chip
.

Also, th
e width of dipole is
chosen

with

1 mm.

Other sizes of the dipole antennas are shown in
Fig. 2.



Fig.
5

T
he return loss of
the

proposed

prototype

antenna and
the
meandered dipole antenna.


By the fine
adjustment of loop size, a
good

impedance
match
ing

was achieved with
w

=
58

mm.
The length of
A
rm
a

is
4

mm and
The length of
Arm
b

is
1
2

mm

for center
frequency operating at 925 MHz
.
Figure
5

shows the
simulated

return loss
es

of
the

constructed prototype

and

the

meander line dipole antenna.

The
i
mpedance bandwidth
determined by
2
:1 VSWR (about
10

dB return loss), reaches
30
MHz

(
910

940

MHz)

or about
3.2
% with respect to the
center frequency
with

925

MHz

and the b
andwidth is
available for RFID system.

800
900
1000
1100
1200
Frequency (MHz)
0
10
20
30
40
Return Loss (dB)

Fig.
6

T
he return loss of
the

constructed prototype

antenna with
h

= 0,
1.6, 3.2 and 4.8 mm.


Fig.
6

plots the
simulated return loss of the proposed
prototype

antenna

with

the different thickness

h

of the
substrate.

Resonate
freque
ncies

are
1160 MHz
,

92
5

MHz
,
868 MHz

and
839 MHz
, which is
related

to
h

=

0, 1.6, 3.2
and 4.8 mm
, respectively.

For compact antenna design,
the
thicker

dielectric substrate is good choice
s
.

Fig.
7

plots the
simulated return loss of the proposed
prototype

a
ntenna

with

the different dielectric constant

r

of the substrate. Dielectric
constant

r

is changed with

r

= 2.2, 4.4, 6.6 and 8.8
, which
resonate
frequencies

are
related

to
1
063

MHz
,

92
5

MHz
,
8
34

MHz

and
769

MHz
, respectively.

That is, t
he larger
dielectric constant, the better choice for compactne
ss of
antenna designs, which is requirement of RFID application.

700
800
900
1000
1100
Frequency (MHz)
0
10
20
30
40
Return Loss (dB)

Fig.
7

T
he return loss of
the

constructed prototype

antenna with

r

=
2.2, 4.4, 6.6 and 8.8
.


Fig.
7

plots the
simulated

radiation patterns at
925

M
Hz
in
three

p
rincipal planes
. Broadside radiation
pattern
with
good
linearly

polarized

radiation is seen

at x
-
z plane, the
larger cross polarization is contributed from the bended arm
of a
dipole

antenna. At
x
-
z plane, t
he
polarization is an
omidirectional field patter
n
.





Fig.
8


Simulated

radiation patterns at
925

MHz for the proposed antenna
.


IV.

C
ONCLUSION

A

prototype constructed for
RFID

operat
ing

in the

922

928

M
Hz

band

are presented

and antenna gain is less 1 dBi
.

Ideal


2

ante
nna gain is about 3 dBi, which is larger than
antenna gain of
the
proposed

antenna
.
Trade off between
gain and compactness is another research
topic

for FRID
application system.


R
EFERENCES

[1]

Dominik Berger
,
Contactless
smart
card standards
and
new
test meth
ods
,

Smart Cart Technology

and application
.

[2]

http://www.junting .com.tw/RFID_inlay_tags>htm#/ALL

.

[3]

C
.

A. Balanis
,
An
tenna

Theory: analysis and design
. New
York:
John Wiley & Sons, Inc.
, 198
2
, ch. 4.